The field of the present disclosure relates to optical readers and methods of data reading, and more particularly, to methods and systems using a small number of high speed scan lines to generate a raster image of an optical code.
Conventional fixed scanners use a motor/facet wheel to scan a laser beam across a plurality of pattern mirrors in order to generate an omnidirectional scan pattern. FIG. 1 illustrates a schematic of a conventional fixed scanner 10. The scanner 10 includes a laser diode 15 generating a laser beam 17 which is directed onto a facet wheel 20. The facet wheel 20 is rotatably driven by a motor 21 at a relatively high speed, typically several thousand rpm. The facet wheel 20 scans the beam across a plurality of pattern mirrors 22 (only one pattern mirror is shown) with the scanned beam reflecting off one or more patterns mirrors to form scan lines projected through a window 24 and into a scan volume.
Return light reflecting off the barcode 5 is collected retrodirectively onto the pattern mirrors 22, the facet wheel 20, and then onto a collection mirror 26 (or alternately a lens) by which it is focused toward a detector 28.
Multiple scan lines are generated forming an omnidirectional scan pattern designed to be capable of scanning a barcode passing through the scan volume in any orientation. An important aspect of scanning efficiency is side coverage, that is, which sides of an item can be scanned, the item being defined as a six-sided cube or six-sided rectangular box-shaped form. L-scanners have been employed to enhance item side coverage. An L-scanner, such as the Magellan® 8500 scanner manufactured by PSC Inc. of Eugene, Oreg., has two windows oriented in a generally “L” shape, one window oriented generally vertically and one window oriented generally horizontally. The Magellan® 8500 scanner has the unique capability of scanning all six sides of an item: (1) the bottom side is scanned primarily by scan lines from the horizontal window; (2) the leading side (i.e. the left side assuming a right-to-left scanning direction) is scanned by scan lines from both the vertical window and the horizontal window; (3) the trailing side (i.e. the right side assuming a right-to-left scanning direction) is scanned by scan lines from both the vertical window and the horizontal window; (4) the front side (i.e. the side facing the vertical window) is scanned by scan lines from the vertical window; (5) the rear or checker side (the side facing opposite the vertical window) is scanned by scan lines from the horizontal window; (6) the top side (the side facing opposite the horizontal window) is scanned by scan lines from the vertical window.
FIG. 2 diagrammatically illustrates a scan pattern 30 generated through the horizontal window of a Magellan® 8500 scanner. The facet wheel of the Magellan® 8500 scanner is rotated at about 100 times per second (6000 rpm). The scan pattern is made of families (or groups) of generally parallel lines, due to the different angular positions of each facet on the facet wheel. For the Magellan® 8500, there are four facets, each arranged at a different angle relative to the rotational axis, so there are four parallel lines with each scan family. There are eight different pattern mirror sets on the horizontal window, leading to 32 scan lines, as shown in the pattern 30 of FIG. 2.
The scan pattern 30 is constrained by the use of families of parallel lines. A relatively large amount of physical space is needed to create the scan pattern. As illustrated in the system 10 of FIG. 1, there is significant depth to the product to contain the pattern mirrors and facet wheel. Depending on the scan engine design, the product may be even deeper or longer to handle collection. The scan lines must emanate from a point laterally beyond the edges of the window, which requires the product to be wider than the window in both dimensions.
Because collection is retrodirective in the typical facet wheel scanner, the facet wheel needs to be quite large. Particularly because of the small number of facets (typically three or four), windage may also be large, causing a large power consumption of the motor/facet wheel assembly. A large facet wheel also produces a significant load on the bearings, affecting the lifetime of the motor. The optical quality of the reflective surfaces of the facet wheel is difficult to maintain, due to the high speed of rotation. In addition, care must be taken to ensure structural integrity of such a facet wheel due to the large stresses from high speed rotation.
Since the scan pattern reads barcodes by spatially covering the window to hit the product at all angles, the window must be fairly large. For scanners with a horizontal window, sapphire or other scratch-resistant surface is used to provide a surface that will last under the harsh environment of products sliding over the window. The cost of this window is quite high and thus is a significant cost factor in the product.